1. Field of the Invention
The present invention relates to a manufacturing method of a solar cell. More particularly, the present invention relates to a method for forming a selective emitter of a solar cell and a diffusion apparatus for forming the same.
2. Description of the Related Art
In recent years, as an environmental pollution problem gets serious, an active research on new and renewable energy for decreasing environmental pollution is conducted. Attention is paid to a solar cell for producing electric energy, particularly, using solar energy among the new and renewable energy. However, in order for the solar cell to be actually applied to industry, a photoelectric conversion efficiency of the solar cell should be high and its manufacturing price should be low.
A description is made in view of the photoelectric conversion efficiency. Because a theoretical limit efficiency of a silicon solar cell is not so high, there is actually a limitation in increasing the photoelectric conversion efficiency of the solar cell. At present, it is reported by a global study group that the silicon solar cell has a photoelectric conversion efficiency of 24% or more.
However, in the case of mass producing the solar cell, it is actually difficult that an average photoelectric conversion efficiency of the solar cell exceeds 17%. Thus, there is a demand for a high efficiency production method applicable to an automation mass production line of a scale of 30 MW or more per year.
In a manufacturing process of a solar cell, an electrode of the solar cell is formed generally by screen printing a metal paste on a substrate. Because of a high contact resistance between a surface of the substrate and the electrode, a photoelectric conversion efficiency of the solar cell may decrease. Thus, in order to reduce the contact resistance, there is a need to form a highly doped region highly doped with impurities in the substrate.
However, if the highly doped region is formed in the whole substrate surface, a cohesion phenomenon occurs because of the existence of excessive impurities. Resultantly, the photoelectric conversion efficiency of the solar cell decreases. Because of this, it is desirable that the highly doped region is selectively formed only in a specific portion of the substrate.
There are a few of conventional methods for forming a selective emitter. The first method includes forming an oxide film pattern on a surface of a substrate by a photolithography process, diffusing impurities in the substrate at a high temperature, and removing the oxide film and diffusing the impurities in the substrate. By twice impurity diffusion, a lowly doped region is formed in a substrate portion covered with the oxide film pattern, and a highly doped region is formed in a substrate portion corresponding to an opening region of the oxide film pattern.
The second method includes forming a silicon oxide film pattern having a partial transmittance on a surface of a substrate, and diffusing impurities in a substrate at a high temperature. In this case, the impurities can partially transmit the silicon oxide film. Thus, a highly doped region is formed in a substrate portion corresponding to an opening region of the silicon oxide film pattern, and a lowly doped region is formed in a substrate portion covered with the silicon oxide film pattern.
However, the above conventional methods have the following problems. In the first method, a manufacturing process of a solar cell is complex and a manufacturing cost of the solar cell increases because a photolithography process, an etching process, a deposition process, etc. are added to form an oxide film pattern on a substrate surface. In addition to this, a photoelectric conversion efficiency of the solar cell manufactured using the first method is not uniform.
In the second method, it is not easy to accurately adjust a transmittance of a silicon oxide film to a desired target value and thus, it is difficult to form a uniform emitter layer in a substrate. A solar cell with the emitter layer having a non-uniform concentration of impurities decreases in its photoelectric conversion efficiency. Also, in the second method, because there is a need for a process of removing an oxide film, etc. after forming a selective emitter, a process time increases. In addition, because the substrate is exposed to an environment of the oxide film removal process, an error rate of the solar cell increases.